1. Technical Field
The present invention generally relates to a technical field of mobile communications, and more particularly to a base station apparatus, a user equipment (UE) terminal, and a communication control method in a mobile communication system using plural antennas.
2. Background Art
Research and development of a next-generation communication system have been continued at high speed. As the next-generation communication system of the W-CDMA (Wideband Code Division Multiple Access) system or the HSDPA (High Speed Downlink Packet Access) system, an LTE (Long Term Evolution) system has been studied by 3GPP (3rd Generation Partnership Project) which is a standards body of the W-CDMA. More specifically, in the LTE system as a radio access system, an OFDM (Orthogonal Frequency Division Multiplexing) scheme and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme have been studied to be applied to the downlink communications system and the uplink communications system, respectively (see, for example, Non-Patent Document 1).
The OFDM scheme is a multi-carrier system in which a frequency band is divided into plural sub-carriers having narrower frequency bands and data are mapped onto the sub-carriers so as to be transmitted. By using the orthogonality among the sub-carriers (i.e., independent from each other), the sub-carriers may partially overlap each other on the frequency axis, thereby enabling improving frequency use efficiency and achieving faster transmission rates.
The SC-FDMA scheme is a single-carrier type transmission system in which a frequency band is divided into plural narrower frequency bands so that the divided frequency bands are allocated to plural user equipment (UE) terminals (mobile stations) so that the user equipment (UE) terminals can transmit using different frequency bands, thereby reducing the interference between user equipment (UE) terminals. Further, in the SC-FDMA scheme, a range of the fluctuation of the transmission power may be made smaller; therefore, lower energy consumption of terminals may be achieved and a wider coverage area may also be obtained.
In both uplink transmissions and downlink transmissions of the LTE system, communications are carried out by allocating one or more physical channels shared among plural user equipment (UE) terminals. The channel shared among plural user equipment (UE) terminals is generally called a shared channel. In the LTE system, uplink communications and downlink communications are carried out by using a Physical Uplink Shared Channel (PUSCH) and a Physical Downlink Shared Channel (PDSCH), respectively.
In the communication system using such a shared channel as described above, it is required to perform signaling (a signaling process) to determine which shared channel is to be allocated to which user equipment (UE) terminal with respect to each sub-frame (having one (1) ms period in the LTE system). The control channel used for the signaling in the LTE system is called a Physical Downlink Control Channel (PDCCH) or a Downlink L1/L2 Control Channel (DL L1/L2 Control Channel). Further, data to be transmitted via the Physical Downlink Control Channel (PDCCH) includes, for example, Downlink Scheduling Information, Acknowledgement/Non-Acknowledgement Information (ACK/NACK), Uplink Scheduling Grant, an Overload Indicator, Transmission Power Control Command Bit and the like (see, for example, Non-Patent Document 2).
The Downlink Scheduling Information and the Uplink Scheduling Grant include information items to be used for the signaling determining which shared channel is to be allocated to which user equipment (UE) terminal. Further, the Downlink Scheduling Information may include information items regarding the Physical Downlink Shared Channel (PDSCH), such as allocation information of downlink Resource Blocks, an ID of a user equipment (UE) terminal (mobile station), the number of streams when MIMO (Multi-Input Multi-Output) communications are performed, information of Pre-coding Matrix, data size, modulation scheme, information of an HARQ (Hybrid Automatic Repeat reQuest) and the like. Further, the Uplink Scheduling Grant may include information items regarding the Physical Uplink Shared Channel (PUSCH), such as allocation information of uplink Resource Blocks, the ID of a user equipment (UE) terminal, the data size, the modulation scheme, uplink transmission power information, information of a Demodulation Reference Signal in an uplink MIMO and the like.
In a MIMO (Multi-Input Multi-Output) communication scheme, plural antennas are used to obtain faster data transmission rates and/or higher quality of the communications. Further, in the MIMO communication scheme, a transmission signal is copied and each of the signals is combined with an appropriate weighting coefficient and transmitted. By doing this, it becomes possible to transmit the signals as controlled beams having directionality. This method is called a Pre-coding method and the weighting factor (or weighting) used in this method is called a Pre-coding matrix.
FIG. 1 schematically shows a case where the Pre-coding is performed. As shown in FIG. 1, each of two (2) streams (Transmission signals 1 and 2) is copied to make two (2) signals so that the two (2) signals travel along two different systems (paths). In each system (path), the signal is multiplied by (combined with) the Pre-coding matrix, so that the combined signal is transmitted. From the viewpoint of utilizing more appropriate Pre-coding matrix (or a set of Pre-coding vectors), the Pre-coding is performed in a closed-loop system as shown in FIG. 1. When the closed loop is formed, the Pre-coding matrix (or a set of Pre-coding vectors) may be adaptively controlled to have more appropriate value based on the feedback of the Pre-coding matrix from the receiving side (user equipment (UE) terminal). In the Pre-coding method, each stream is spatially separately transmitted; therefore, a greater quality improvement with respect to each stream may be obtained.                Non-Patent Document 1: 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA,” June 2006        Non-Patent Document 2: 3GPP R1-070103, “Downlink L1/L2 Control Signaling Channel Structure: Coding”        